Ethan, I've been thinking.Ethan Winer wrote:I have to add a qualifier to that advice, though I don't know the best way to optimize the mass / spring ratio. Maybe you do?AVare wrote:They will yield about the same absorption. The absorption is result of the sum of the mass and the depth of the air. Either will work the same.
There is an ideal balance between the mass and spring. I believe it's when both have the same impedance at the center frequency. The low-bass panel trap on my web site uses 1/4 inch plywood with a 4-inch gap to give a center frequency around 100 Hz. When I tested traps of that design at IBM's lab, the absorption peaked at 100 percent. So we can take that as known to work efficiently.
You can lower the center frequency one octave by using a gap twice as large. But the tuned system will not be as efficient. As an extreme example, imagine 1/4 plywood with a 40-inch gap. It should be intuitively obvious that this will not absorb 10 Hz efficiently.
A tuned LC (inductor / capacitor) circuit is very similar to a mass / spring absorber. You can tune the circuit to a wide range of frequencies by changing either the inductor, the capacitor, or both. But the Q and efficiency also vary. With LC electronic circuits it's easier (for me) to determine when the inductor and capacitor impedances are the same. I'm sure there's a way to do this with panel traps! I just don't know how.
--Ethan
If the 1/4" thick plywood panel together with a 4" deep spring yields approx 100 hz center frequency according to your tests, it doesn't hold up to the formula presented in Rods book.(F = 170/sqrt (m x d)). I'm calculating on a 7 mm(approx 1/4" thickness) together with 5" airgap, and i'm achieving around 50 hz, how could that 1" depth yield a lowering of the center frequency of 50 hz? must be something strange going on here.